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NEW YORK (Reuters Health) - If milk does the heart good, it might do the heart better if it comes from dairy cows grazed on grass instead of on feedlots, according to a new study.

Earlier experiments have shown that cows on a diet of fresh grass produce milk with five times as much of an unsaturated fat called conjugated linoleic acid (CLA) than do cows fed processed grains. Studies in animals have suggested that CLAs can protect the heart, and help in weight loss.

Hannia Campos of the Harvard School of Public Health in Boston and her colleagues found, in a study of 4,000 people, that people with the highest concentrations of CLAs -- the top fifth among all participants -- had a 36 percent lower risk of heart attack compared to those with the lowest concentrations.

Those findings held true even once the researchers took into account heart disease risk factors such as high blood pressure and smoking.

The new findings suggest that CLA offers heart-healthy benefits that could more than offset the harms of saturated fat in milk, Campos said.

"Because pasture grazing leads to higher CLA in milk, and it is the natural feed for cattle, it seems like more emphasis should be given to this type of feeding," she told Reuters Health by email.

Dairy products in the U.S. come almost exclusively from feedlots, she added. And cow's milk is the primary source of CLA. (Beef contains a small amount.)

Campos and her colleagues looked to Costa Rica for their study, where pasture grazing of dairy cows is still the norm. They identified nearly 2,000 Costa Ricans who had suffered a non-fatal heart attack, and another 2,000 who had not. Then they measured the amount of CLA in fat tissues to estimate each person's intake.

Since CLA typically travels with a host of other fats, the researchers went a step further to tease apart its effects from those of its predominantly unhealthful companions, they report in the American Journal of Clinical Nutrition. The difference in risk attributed to CLA subsequently rose to 49 percent.

"Whole-fat milk and dairy products have gotten such a bad reputation in recent years due to their saturated fat and cholesterol contents, and now we find that CLA may be incredibly health-promoting," Michelle McGuire, spokesperson for the journal's publisher, the American Society for Nutrition, and associate professor at Washington State University, told Reuters Health in an email. "Whole milk is not the villain!"

Each year, approximately 1.5 million Americans will suffer a heart attack. A third will not survive.

The evidence may now be piling up: another paper out of Sweden in the same issue of the journal as the Costa Rican study also hints at heart attack protection through milk fat.

Further, the benefits of CLA may extend beyond the heart to the prevention of cancer and diabetes, suggests McGuire, pointing to results of other animal studies. "Milk is actually the only food ever 'designed by nature' to be fed to mammals," she added. "We need to look to milk as the perfect food and learn everything we can from it."